As the floor is just a cheap sandwich of very thin plywood with Styrofoam in the middle, you can't do much with that. So the only solution seems to be to lay something on top, running across the floor "joists".
Since it is in the bathroom, you could go with a thicker laminate floor. Just check with your flooring guys for the most moisture-resistant stuff they got. Since it is such a small area, you can afford to buy top quality flooring.